Debrominative chlorination of pyrroles

ABSTRACT

There is provided a simple quantitative procedure for the chlorination of pyrrole compounds containing electron withdrawing substituents via the displacement of bromine with chlorine on the pyrrole nucleus.

This is a continuation-in-part of copending application Ser. No. 07/983,204 filed on Nov. 30, 1992.

BACKGROUND OF THE INVENTION

A wide variety of pyrrole compounds substituted with one to three chlorine atoms on the pyrrole ring are highly useful as fungicidal, insecticidal, molluscicidal, acaricidal and nematocidal agents. However, chlorination of these pyrrole compounds frequently proves to be difficult due to over chlorination and/or oxidation of the pyrrole ring.

It is an object of this invention to provide a simple quantitative procedure for introducing chlorine substituent(s) to pyrrole compounds containing electron withdrawing groups. It is another object of this invention to provide a convenient and readily available source of chlorinated pesticidal pyrrole compounds.

SUMMARY OF THE INVENTION

This invention relates to a process for the preparation of a chlorinated pyrrole compound comprising reacting a pyrrole compound containing at least one electron withdrawing substituent and at least one hydrogen or bromine substituent with an effective amount of a chlorinating agent in the presence of a solvent, and when the pyrrole compound containing at least one electron withdrawing substitutent has no bromine substitutents on the pyrrole ring, in the presence of an external or exogenous catalytic amount of bromine. One embodiment of this invention relates to a process for the preparation of a chlorinated pyrrole compound of formula I ##STR1## wherein W is CN, NO₂, CO₂ R₁, CONR₂ R₃, CSNR₄ R₅ or S(O)_(n) CF₂ R₆ ;

X is Cl or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups;

Y is CF₃, Cl or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups;

Z is C₁ or CF₃ with the proviso that at least one of X, Y or Z must be C₁ ;

A is hydrogen or C₁ -C₆ alkyl;

R₁ is C₁ -C₆ alkyl, C₃ -C₆ cycloalkyl or phenyl;

R₂, R₃, R₄ and R₅ are each independently hydrogen, C₁ -C₄ alkyl, C₁ -C₄ haloalkyl, or phenyl optionally substituted with one or more Cl, Br, I, F, NO₂, CN, C₁ -C₄ alkyl, C₁ -C₄ haloalkyl, C₁ -C₄ alkoxy or C₁ -C₄ haloalkoxy groups;

R₆ is hydrogen, F, CL Br, CF₂ H, CCl₂ H, CF₃ or CCl₃ ;

R₇ is hydrogen F, CHF₂, CHFCl or CF₃ ;

R₈ is hydrogen or C₁ -C₄ alkyl;

R₉ is hydrogen or C₁ -C₄ alkyl;

R₁₀ is hydrogen, C₁ -C₄ alkyl or R₁₁ CO;

R₁₁ is hydrogen or C₁ -C₄ alkyl;

B is S(O)_(m) or oxygen; and

m and n are each independently an integer of 0, 1 or 2;

which comprises reacting a compound of formula II ##STR2## wherein L is hydrogen, Br or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups;

M is hydrogen, Br, CF₃ or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups;

R is hydrogen, Br or CF₃ with the proviso that at least one of L, M or R must be Br; and

W, A, R₇, R₈, R₉ and R₁₀ are as described above for formula I;

with an effective amount of a chlorinating agent in the presence of a solvent.

Pyrrole compounds of formula I are useful as insecticides, molluscicides, acaricides, nematocides and fungicides.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a simple procedure for the introduction of chlorine to a pyrrole ring via the displacement of the appropriate bromopyrrole precursor. Direct chlorination of the pyrrole ring via the displacement of hydrogen is frequently difficult. Such reactions often result in low yields of the desired product and the generation of undesireable side-products due to over chlorination and ring oxidation. It has now been found that debrominative chlorination, the displacement of one or more bromine atoms on the pyrrole ring by chlorine, proceeds in a simple quantitative manner, thereby facilitating the chlorination of electron deficient pyrroles under mild reaction conditions to give the desired product in high purity and yield.

One embodiment of this invention relates to a process for the preparation of a chlorinated pyrrole compound of formula I ##STR3## wherein W is CN, NO₂, CO₂ R₁, CONR₂ R₃, CSNR₄ R₅ or S(O)_(n) CF₂ R₆ ;

X is Cl or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups;

Y is CF₃, Cl or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups;

Z is Cl or CF₃ with the proviso that at least one of X, Y or Z must be Cl;

A is hydrogen or C₁ -C₆ alkyl;

R₁ is C₁ -C₆ alkyl, C₃ -C₆ cycloalkyl or phenyl;

R₂, R₃, R₄ and R₅ are each independently hydrogen, C₁ -C₄ alkyl, C₁ -C₄ haloalkyl or phenyl optionally substituted with one or more Cl, Br, I, F, NO₂, CN, C₁ -C₄ alkyl, C₁ -C₄ haloalkyl, C₁ -C₄ alkoxy or C₁ -C₄ haloalkoxy groups;

R₆ is hydrogen, F, Cl, Br, CF₂ H, CCl₂ H, CF₃ or CCl₃ ;

R₇ is hydrogen, F, CHF₂, CHFCl or CF₃ ;

R₈ is hydrogen or C₁ -C₄ alkyl;

R₉ is hydrogen or C₁ -C₄ alkyl;

R₁₀ is hydrogen C₁ -C₄ alkyl or R₁₁ CO;

R₁₁ is hydrogen or C₁ -C₄ alkyl;

B is S(O)_(m) or oxygen; and

m and n are each independently an integer of 0, 1 or 2;

which comprises reacting a compound of formula II ##STR4## wherein L is hydrogen, Br or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups;

M is hydrogen, Br, CF₃ or phenyl optionally substitued with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups;

R is hydrogen, Br or CF₃ with the proviso that at least one of L, M, or R must be Br; and

W, A, R₇, R₈, R₉ and R₁₀ are as described above for formula I;

with a chlorinating agent in the presence of a solvent.

In another embodiment of the invention, the appropriate bromopyrrole precursor may be formed in situ by chlorinating a pyrrole compound with no bromine substituents on the pyrrole ring in the presence of an external or exogenous catalytic amount of bromine. Typically, the catalytic amount of bromine ranges between about 0.01 and 1 mole %, about 0.1 mole % depending on the specific reaction conditions. An example is shown in flow diagram I, wherein A and W are described hereinabove. ##STR5##

In the first embodiment of the invention the desired chloropyrrole product may be prepared by the chlorination of the already formed and isolated bromopyrrole precursor. Two examples are shown in flow diagrams II and III. ##STR6##

Compounds of formula II, which are useful as reactants in the method of the invention, and methods for their preparation, including typically useful reagents, are described in U.S. Pat. Nos. 5,010,098, 5,008,403 and 5,162,308 among other publications.

In general, the external or exogenous catalytic use of bromine is more suitable for those pyrrole compounds of formula II wherein A is hydrogen and L, M and R are other than bromine. In those instances wherein A is C₁ -C₆ alkyl, debrominative chlorination may take place when the pyrrole compound contains at least one hydrogen or bromine substituent.

An effecive amount of chlorinating agent may be the theoretical amount required to obtain the desired chlorinated pyrrole product of formula II, i.e., 1 molar equivalent for a monochlorinated product, 2 molar equivalents for a dichlorinated product or 3 molar equivalents for a trichlorinated product. Of course, it is contemplated that an excess of theoretical amount may also be an effective amount of chlorinating agent.

Chlorinating agents known in the art such as chlorine gas, N-chlorosuccinimide, sulfuryl chloride, and the like are suitable for use in the method of invention.

Solvents which are unreactive toward chlorinating agents may be used in the method of invention, preferably those which are known to be suitable for chlorination reactions such as carbon tetrachloride, acetic acid, and haloaromatics such as chlorobenzene and the like.

The simple, quantitative method disclosed herein of introducing chlorine atoms to a pyrrole ring containing electron withdrawing substituent(s) may take place under mild reaction conditions such as room temperature. However, the reaction rate is proportional to the reaction temperature and, therefore, increased reaction temperatures give increased reaction rates and decreased reaction times. However, sharp increases in reaction temperature may lead to a decrease in desired product yield due to undesirable side-reactions and decomposition.

Flow diagrams I, II and III hereinabove provide some illustration of the wide variety of chlorinated pyrrole products which may be prepared by the inventive method. In addition, the following specific examples are set forth below for a more clear understanding of the invention. The examples are merely illustrative and are not be be understood as limiting the scope and underlying principles of the invention in any way. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the following examples and the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.

EXAMPLE 1 Preparation of 4-Chloro-2-(p-chlorophenyl)-5-(trifluoromethyl)pyrrole-3-carbonitrile ##STR7##

A mixture of 2-(p-chlorophenyl)-5-(trifluoromethyl)-pyrrole-3-carbonitrile (8.12 g, 0.03 mol) and sodium acetate (0.25 g, 0.003 mol) in acetic acid at 70° C. is treated dropwise with bromine (0.48 g, 0,003 mol). When the color disappears, the reaction is treated dropwise with sulfuryl chloride (6.1 g, 0.045 mol) over a 1 hour period, held at 70° C. for 1 hour, concentrated in vacuo and diluted with water. The reaction mixture is filtered, the filter cake is washed with water and dried to yield the title product as white solid, 7.7 g (84% yield), mp 242°-243° C.

EXAMPLE 2 Preparation of 4-Chloro-2-(3,4-dichlorophenyl)-5-(trifluoromethyl)pyrrole-3-carbonitrile ##STR8##

Using essentially the same procedure described in Example 1 and substituting 2-(3,4-dichlorophenyl)-5-(trifluoromethyl)pyrrole-3-carbonitrile as the starting material, the title product is obtained in 69% yield as a white solid, mp 239°-241° C.

EXAMPLE 3 Preparation of 4-Bromo-2-(p-chlorophenyl)-1-methyl-5-(trifluoromethyl)pyrrole-3-carbonitrile ##STR9##

A solution of 2-(p-chlorophenyl)-1-methyl-5-(trifluoromethyl)pyrrole-3-carbonitrile (5.0 g, 0.0176 mol) in carbon tetrachloride is treated with bromine (56 g, 0.0351 mol), heated at reflux temperature for 8 hours, cooled, washed sequentially with water, aqueous sodium metabisulfite and water, dried over sodium sulfate and concentrated in vacuo to give a solid residue. The residue is recrystallized from heptane to give the title produce as a white solid, 6.0 g (94% yield), mp 126°-129° C.

EXAMPLE 4 Preparation of 4-Chloro-2-(p-chlorophenyl)-1-methyl-5-(trifluoromethyl)pyrrole-3-carbonitrile ##STR10##

A solution of 4-bromo-2-(p-chlorophenyl)-1-methyl-5-(trifluoromethyl)pyrrole-3-carbonitrile (20.0 g, 0.055 mol) in chlorobenzene at 60° C. is treated with chlorine gas at 60° C. over a 1/2 hour period, cooled to room temperature, washed sequentially with water, aqueous sodium metabisulfite and water, dried over sodium sulfate and concentrated in vacuo to give a residue. The residue is crystallized in heptane to give the title product as a pale yellow solid, 17.6 g (quantitative yield), mp 109.5°-110.5° C.

EXAMPLE 5 Preparation of 1-Methyl-2,4,5-trichloropyrrole-3-carbonitrile ##STR11##

A mixture of 1-methyl-2,4,5-tribromopyrrole-3-carbonitrile (40.0 g, 0.117 mol) in chlorobenzene is heated to 40° C., treated with chlorine gas at a rate sufficient to maintain a reaction temperature below 35° C. while using ice-bath cooling. When addition is complete, the reaction mixture is cooled to room temperature, washed sequentially with water, sodium metabisulfite and water, dried over sodium sulfate and concentrated in vacuo to give a residue. The residue is crystallized in heptane to give the title product as a white solid, 22.8 g (93% yield), mp 113°-114° C.

EXAMPLE 6 Preparation of 4,5-dichloro-2-(3,4-dichlorophenyl)-pyrrole-3-carbonitrile ##STR12##

A slurry of 5-bromo-2-(3,4-dichlorophenyl)pyrrole-3-carbonitrile (1.58 g, 0.005 mol) in glacial acetic acid (40 mL) is warmed to 50° C., treated dropwise with sulfuryl chloride (1.62 g, 0.012 mol, 2.4 molar equivalents), heated at 80° C. for 1.5 hours, cooled to room temperature and poured into water. The reaction mixture is extracted with ethyl acetate and the extracts are combined and concentrated in vacuo to give a residue. The residue is purified by flash chromatography using silica gel, packed and eluted with 20% ethyl acetate in heptane, to give the product as a white solid. The title product is identified by HPLC¹ and mass spectral analyses. 

I claim:
 1. A process for the preparation of a chlorinated pyrrole compound which comprises reacting a pyrrole compound containing at least one electron withdrawing substituent and at least one bromine substituent with an effective amount of a chlorinating agent in the presence of a solvent whereby chlorine displaces the at least one bromine substituent.
 2. The process according to claim 1 wherein said chlorinated pyrrole compound has structural formula I ##STR13## wherein W is CN, NO₂, CO₂ R₁, CONR₂ R₃, CSNR₄ R₅ or S(O)_(n) CF₂ R₆ ;X is Cl or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups; Y is CF₃, Cl or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups; Z is Cl or CF₃ with the proviso that at least one of X, Y or Z must be Cl; A is hydrogen or C₁ -C₆ alkyl; R₁ is C₁ -C₆ alkyl, C₃ -C₆ cycloalkyl or phenyl; R₂, R₃, R₄ and R₅ are each independently hydrogen, C₁ -C₄ alkyl, C₁ -C₄ haloalkyl, or phenyl optionally substituted with one or more Cl, Br, I, F, NO₂, CN, C₁ -C₄ alkyl, C₁ -C₄ haloalkyl, C₁ -C₄ alkoxy or C₁ -C₄ haloalkoxy groups; R₆ is hydrogen, F, Cl, Br, CF₂ H, CCl₂ H, CF₃ or CCl₃ ; R₇ is hydrogen F, CHF₂, CHFCl or CF₃ ; R₈ is hydrogen or C₁ -C₄ alkyl; R₉ is hydrogen or C₁ -C₄ alkyl; R₁₀ is hydrogen, C₁ -C₄ alkyl or R₁₁ CO; R₁₁ is hydrogen or C₁ -C₄ alkyl; B is S(O)_(m) or oxygen; and m and n are each independently an integer of 0, 1 or 2;which comprises reacting a compound of formula II ##STR14## wherein L is hydrogen, Br or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups; M is hydrogen, Br, CF₃ or phenyl optionally substituted with one to three C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylthio, C₁ -C₄ alkylsulfinyl, C₁ -C₄ alkylsulfonyl, F, Cl, Br, I, CN, NO₂, CF₃, R₇ CF₂ B, R₈ CO or NR₉ R₁₀ groups; R is hydrogen, Br or CF₃ with the proviso that at least one of L, M or R must be Br; and W, A, R₇, R₈, R₉ and R₁₀ are as described above with an effective amount of a chlorinating agent in the presence of a solvent.
 3. The process according to claim 2 wherein the solvent is carbon tetrachloride.
 4. The process according to claim 3 wherein A is C₁ -C₆ alkyl.
 5. The process according to claim 2 wherein the chlorinating agent is selected from the group consisting of chlorine gas, sulfuryl chloride and N-chlorosuccinimide.
 6. The process according to claim 2 for the preparation of a compound of formula I wherein W is CN, X is Cl, Y is Cl or phenyl optionally substituted with one to three Cl, Br, I or CF₃ groups and Z is CF₃ or Cl.
 7. The process according to claim 6 wherein A is methyl. 